Semiconductor image sensors are operated to sense light. Typically, the semiconductor image sensors include complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors, which are widely used in various applications such as digital still camera (DSC), mobile phone camera, digital video (DV) and digital video recorder (DVR) applications. These semiconductor image sensors utilize an array of image sensor elements, each image sensor element including a photodiode and other elements, to absorb light and convert the sensed light into digital data or electrical signals.
A front side illuminated (FSI) CMOS image sensor and a back side illuminated (BSI) CMOS image sensor are two types of CMOS image sensors. The FSI CMOS image sensor is operable to detect light projected from its front side while the BSI CMOS image sensor is operable to detect light projected from its backside. The BSI CMOS image sensor can shorten optical paths and increase fill factors to improve light sensitivity per unit area and quantum efficiency, and can reduce cross talk and photo response non-uniformity. Hence, the image quality of the CMOS image sensor can be significantly improved. Furthermore, the BSI CMOS image sensor has a high chief ray angle, which allows a shorter lens height to be implemented, so that a thinner camera module is achieved. Accordingly, the BSI CMOS image sensor technology is becoming a mainstream technology.
However, conventional BSI CMOS image sensors and methods of fabricating the BSI CMOS image sensors have not been entirely satisfactory in every aspect.